The attachment of components to printed circuit boards (PCBs) produces printed circuit board assemblies (PCBAs), which can be used as motherboards in computers such as blades and servers, in mid-planes, as cards such as graphics cards, and for other purposes. A PCB is a laminated board made of an insulating material such as plastic which contains several layers of metal such as copper separated by insulating material. The metal may function to establish electrical connections between parts mounted on the board, conduct heat, or provide a ground.
One increasingly popular component of PCBAs is a quad flat pack no-lead (QFN) chip. QFNs can also be called MicroLead Frames (MLF). A QFN is an electronic component encapsulated in plastic or some other insulating material. A QFN contains a row of IO pads, areas in which bare metal is exposed, on each of its four sides (hence, the “quad” in QFN) for electrical connectivity with the PCB. The QFN also typically contains a thermal pad underneath, an exposed area of metal for conducting heat away from the package. A QFN may be light, present a small footprint, and feature good thermal and electrical conductivity. The small footprint conserves space on the PCB, which can be scarce.
Good thermal conductivity helps to maintain the QFN and the point of connection at an acceptable temperature, thus preserving the useful life and reliability of the chip. A QFN can be attached to a PCB by soldering it directly to a PCB. QFNs may prove more difficult to attach to PCBs than components with leads. Those components may be attached to a PCB by soldering the lead to the PCB. Soldering together two flat planes, the QFN thermal pad and the PCB, may be more difficult than soldering a lead from a leaded component to the PCB. To solder the QFN to the PCB, solder paste, which may contain solder and flux chemicals, can be applied to the surface of the PCB at appropriate regions. The solder paste can be applied to the PCB surface by extrusion through a stencil. The solder paste can be placed on the stencil and forced through the apertures of the stencil by pressing with a squeegee. After the application of solder paste, the QFN can be positioned on the PCB, and the assembly placed into an oven or series of ovens and heated. The heating can evaporate the flux chemicals and other solvents and cause the solder to melt, leading to wetting and wicking. A solder mask can also be placed on the PCB to control the solder paste during heating. The solder mask defines openings on the outer layers of the PCB and exposes the copper features of the PCB. The solder mask helps to prevent the liquid solder from flowing away from the desired areas of solder application. The solder mask is placed over the PCB, and solder paste is applied to areas of the PCB to which the QFN is to be attached that are not protected with a solder mask.
To prepare for attachment of a QFN, the area of the PCB on which the QFN will rest may be fitted with IO pads and a thermal pad, regions for contact with the QFN IO pads and QFN thermal pad. The pads may consist of copper or another metal. When the QFN is attached to the PCB, the QFN pads rest on the corresponding PCB pads and are connected with solder. The PCB pads may be slightly larger than the QFN pads to provide tolerance for imperfect placement. The QFN IO pads may be soldered to the PCB IO pads to provide an electrical connection between the PCB and the QFN. The thermal pad of the QFN may be soldered to the PCB thermal pad to provide thermal conductivity and a mechanical connection and can also provide an electrical connection. The direct soldering of bare metal areas of the QFN to the surface of the PCB may provide for good electrical and thermal conductivity as well as a good mechanical connection. To conduct away heat transferred from the QFN thermal pad to the PCB thermal pad, the PCB thermal pad region may contain vias. Vias are conduits which may connect the PCB thermal pad to metal layers in the interior of the PCB and on the opposite surface of the PCB. The vias may consist of metal-plated tunnels through the PCB. The vias may be connected to the QFN only indirectly. Instead of a direct connection, the thermal pad of the QFN may be soldered to the PCB thermal pad, and the PCB thermal pad may be connected to the vias.
Generally, solder does not cover the entire PCB thermal pad. Instead, smaller regions of solder, called solder pads, can be deposited on the PCB thermal pad. The amount of coverage may be expressed as a percentage. For example, 50% coverage indicates that half the area of the thermal pad is covered with solder. Solder is not placed on the vias. The solder can damage the vias. The amount of solder placed on the PCB thermal pad is critical to the attachment process. When too much solder is placed on the PCB thermal pad, the QFN may actually float on top of the solder. The heating process may create a ball of solder in the middle of the QFN on which the QFN floats. The QFN may begin to turn, depending on how the QFN was placed on the PCB and on other factors such as air movement and vibration. The movement of the QFN may create shorts in the QFN IO pads. Movement of the QFN IO pads may cause solder to smear from one QFN IO pad to another. Further, the floating of the QFN on the excess solder in the thermal pad region can interfere with the forming of solder joints in the IO pad regions. On the other hand, too little solder can cause a poor or non-existent connection between the QFN and the PCB.
In addition to the total amount of solder, the size of the individual solder pads affects the attachment process. A solder pad with minimum diameter smaller than the width of a stencil through which the solder pad is extruded may not deposit out of the stencil in the proper shape. Regions of solder with minimum dimension equal to 1.5 times the width of the stencil may provide for the more reliable deposit of solder in the shape of the holes in the stencil. Solder pads that are too large may create voiding. Gas may be trapped underneath a large pad, preventing the connection between the solder and the QFN. Voiding is much less a problem with smaller sizes of solder pads. The gases can escape to the sides of the solder pads.
The guidance offered by QFN manufacturers is often not helpful. The advice may be contradictory. It may call for the placement of a large number of vias on the thermal pad and a percentage coverage of the thermal pad with solder that is impossible to achieve given the number of vias. The advice may call for excess amounts of solder. Attempting to follow the manufacturers' recommendations for the amount of solder has led to large rejection rates of PCBAs, leading to the expense and time of reworking the PCBAs.